Recently, there has been an increasing demand for flat panel display (FPD) devices. Especially, organic EL display devices using an organic EL (Electro Luminescence) element (OLED; Organic Light Emitting Diode) are excellent in power consumption, lightweightedness, thinness, moving image characteristics, viewing angle and the like, and are now being progressively developed and put into practice. An organic EL display device includes a pixel circuit including a driving transistor. The driving transistor of the pixel circuit controls a driving current flowing in an organic EL element in accordance with a video voltage that is in accordance with a video data that is input to a gate electrode of the pixel circuit, and thus controls the gray scale of an image to be displayed. In general, a driving transistor is a polysilicon thin film transistor that uses polysilicon (polycrystalline silicon) for a semiconductor film. Regarding such a polysilicon thin film transistor, it is known that the variance in the threshold voltage is large or that the threshold voltage varies along with time. Therefore, an organic EL display device in which the gray scale is controlled by use of the video voltage in accordance with the video data has a problem that the variance in the threshold voltage of the driving transistor or the change in the threshold value along with time changes the value of the current flowing in the organic EL element, which causes a variance in the luminance. Patent Document 1 (Japanese Laid-Open Patent Publication No. 2009-169432) describes that in order to solve the above-described problems, the value of the current flowing in the organic EL element in each of pixels is detected, and a predetermined offset voltage is applied based on the detected value to correct the threshold voltage.
According to a conventionally known driving method used to compensate for the threshold voltage of a driving transistor as described above, an initialization voltage and a video voltage are applied alternately. When this technique is used, the time period which can be used to write a video voltage to each of pixels is shortened to about half. In the case of a square structure including red (R), green (G), blue (B) and white (W) pixels, the time period usable to write a video voltage to each of the pixels is further shortened to half. The time period usable to write a video voltage to each of the pixels is also shortened when the number of scanning lines is increased in order to realize high definition display.
In order to write a video voltage to each pixel within a short time period, the resistance and the capacitance of video (source) lines or the like need to be reduced. However, this is difficult because the width of the lines is decreased and the number of intersections of the lines is increased in order to realize high definition display.